As well known, a phosphor layer is provided on the inner surface of a glass tube for low pressure type fluorescent discharge lamps, and on the inner surface of an outer glass tube having a light emitting tube accommodated therein for the high pressure type lamps.
In fluorescent lamps which are representative of low pressure type fluorescent discharge lamps, a greater part of ultraviolet rays generated by means of an electric discharge of a mercury vapor is absorbed by the phosphor layer to be converted to light of a long wavelength. One part of the light passes through the phosphor layer to be absorbed by glass, resulting in a loss (an absorption loss), while another part thereof is relfected from the phosphor layer and absorbed by the electric discharge, resulting in a further loss (a reflection loss). Also, in the high pressure type fluorescent discharge lamps such as high pressure mercury fluorescent lamps, members exist for absorbing ultraviolet rays such as glass and the light emitting tube other than the fluorescent layer, to cause an absorbtion and a reflection loss such as described above.
In order to improve the light output from such fluorescent discharge lamps, it is desirable to decrease the absorption and reflection losses and absorb ultraviolet rays generated with electric discharges by the phosphor layer as much as possible. As a method of decreasing the absorption and reflection losses, it is known to stack a plurality of phosphor layers on a glass substrate in such a manner that the layer located nearest to the electric discharge side is composed of phosphor particles having a low reflection factor to ultraviolet rays. According to Japanese patent publication No. 32,959/1975 there is disclosed the fact that, upon stacking a plurality of phosphor layers having different reflection factors to ultraviolet rays, phosphor particles low in reflection factor to ultraviolet rays have a large mean particle diameter, while phosphors high in reflection factor to ultraviolet rays have a small mean particle diameter.
In order to constitute the phosphor layers in this way, it is necessary to separately provide a phosphor having a small mean particle diameter and that having a large mean particle diameter in substantially equal amounts, and also it is required that there is a large difference in mean particle diameter between the two. According to follow-up experiments of the inventors, however, a phosphor powder normally synthesized has a small proportion of particles having the large and small mean particle diameters required for such phosphor layers and when the powder is separated by means such as elutriation or the like, there is a large amount of undesirable particles having intermediate mean particle diameters. Discarding the undesirable particles is not considered in mass production systems, and therefore when an attempt is made to pulverize them by a grinder such as a ball mill. For use as particles of a small mean particle diameter, the destruction of the phosphor proceeds by means of the so-called pressure disruption in the pulverizing step to decrease the quantum yield (ratio of the number of emitting quanta to that of absorbed quanta, that is, a quantum yield upon conversion of a wavelength). This increases the loss in energy. Thus it has been found that, even if the phosphor layers were stacked into the abovementioned construction, the desired lamp efficiency is not obtained.
Thus, the present inventors have examined the provision of phosphors high in reflection factor to ultravoilet rays and also high in quantum yield, and it has been found that if the concentration of an activator is changed to adjust the reflection factor to ultraviolet rays, then the quantum yield can be improved.
This phenomenon will be described as follows:
Phosphors used with electric discharge lamps are, in many cases, composed of a matrix and activator. For example, in trivalent terbium activated yttrium silicate [Y.Tb).sub.2 SiO.sub.5 ] described in Japanese patent publication No. 37,670/1973, the yttrium silicate (Y.sub.2 SiO.sub.5) is the matrix and the terbium (Tb) is an activator.
The Table below takes trivalent terbium activated yttrium silicate phosphor as an example and indicates changes in reflection factor to ultraviolet rays and quantum yield (relative value) when the concentration of the activator, terbium (Tb), is changed. This phosphor provides the highest luminescence output with ultraviolet excitation when it includes 0.16 gram atom of terbium (Tb) with respect to substantially 0.84 gram atom of yttrium. Thus for use with electric discharge lamps, this concentration of the activator is normally adopted. In a Table, Nos. 1 to 5 have the mean particle diameter (10 microns) on the order of that normally used, and are merely changed in concentration of the activator, terbium (Tb). No. 6 has the same concentration of the activator as No. 5 but has the mean particle diameter decreased to 2.7 microns by means of a grinder such as a ball mill or the like. As shown in the Table, a reduction in concentration of the activator causes an increase in reflection factor to an ultraviolet ray (a decrease in amount of absorption of the ultraviolet ray) and improvement in quantum yield. Furthermore, by comparing No. 1 and No. 6 having the same reflection factors to the ultraviolet ray, it is found that a far more advantageous quantum yield is obtained when the reflection factor is adjusted by changing the concentration of the activator, than when it is done by changing the mean particle diameter through the pulverization.
TABLE ______________________________________ Reflection Relative Mean Factor to Lumines- Relative Composition Particle Ultravi- cence Quantum of Diameter olet Ray Output Effi- NO Phosphor (microns) (254nm) (%) ciency ______________________________________ 1 (Y0.96Tb0.04).sub.2 10 0.40 74 1.00 SiO.sub.5 2(Y0.93Tb0.07).sub.2 10 0.25 91 0.98 SiO.sub.5 3(Y0.90Tb0.10).sub.2 10 0.19 97 0.97 SiO.sub.5 4(Y0.87Tb0.13).sub.2 10 0.15 99 0.94 SiO.sub.5 5(Y0.84Tb0.16).sub.2 10 0.13 100 0.93 SiO.sub.5 6(Y0.84Tb0.16).sub.2 2.7 0.40 67 0.91 SiO.sub.5 ______________________________________
In this Table the reflection factor to the ultraviolet ray designated its value when MgO is made 1.00.